Can't assign DateTime with only year component

Transferred from internal JIRA issue originally created 2023-09-20.

A four-digit number is a valid DateTime, but it matches the NUMBER token, so it doesn’t get assigned correctly:
```
* valueDateTime = 2023
```
When trying to assign a number to a DateTime, check if the raw value of the assignment is a four-digit number. If so, allow it.

No file or test is named. Start by locating the assignment and type-parsing path that handles NUMBER tokens, reproduce `valueDateTime = 2023`, and verify that a four-digit raw value is accepted as a DateTime without breaking numeric assignments.

Written by the indexing model from the issue text.